1. Field of the Invention
The present invention relates to the field of combusting fuels with the aim of supplying heat to thermal power plants and/or for other purposes.
More particularly, the present invention relates to a combustion apparatus including a combustion chamber, a source of air and a flue gas recirculation system and methods for making and using same.
2. Description of the Related Art
In the process of the combustion of fuels, a minimum quantity of air, that is theoretically necessary for complete combustion of the fuel, is such that all oxygen contained in the air supplied to the combustion is completely consumed or used. Such a process is referred as a stoichiometric process, and corresponds to the highest temperature in the combustion zone given a combustion process that uses air as the oxidant.
However, in actual practice, all current industrially used combustion systems operate with some excess of air, i.e., an amount of air in excess of the stoichiometric amount, that is necessary to assure the complete combustion of the fuel. This excess air results in a lowering of the temperature of combustion. The greater the excess of air, the lower the temperature of combustion and the less heat available for conversion to a useable from of energy.
However, the greater the excess of air, the greater the flow rate of the produced flue gases. Because the flue gas cannot be cooled to a temperature equal to the initial temperature of the supplied air, the quantity of heat rejected into the atmosphere by the flue gas increases with increasing amounts of excess air. This results in a reduction of the efficiency of the combustion system.
Therefore, in conventional combustion systems, in order to operate with a minimum of excess of air, the tubes in which the boiling of a working fluid of a power cycle occurs, (so-called “waterwall” tubes), are located directly in the combustion zone. This allows the heat of combustion to be partially absorbed by the boiling of the working fluid, and thus controls the temperature in the combustion zone. Such systems are known as conventional boiler combustion systems. These systems are, perforce, expensive and complex structures that require a high degree of maintenance, especially due to the fact that the waterwall tubes are subjected to very high thermal stresses.
On the other hand, in so-called fluidized bed combustors, (which have several advantages), the excess of air is usually very high due to the fact that there is a substantial flow of air needed to maintain the fluidized bed. As a result, fluidized bed boiler/combustors have substantially reduced efficiencies.
In general, it would be extremely desirable, and would present a great simplification, if combustion were to be performed in a separate combustion chamber without the need for internal cooling by waterwall tubes, while at the same time operating with a minimum of excess air. All heat produced by the combustion would thus be accumulated in a stream of hot flue gas which could then be utilized in a heat recovery steam generator (HRSG) or a heat recovery vapor generator (HRVG). HRSG and HRVG systems are relatively simple heat exchangers which are substantially less expensive than conventional boilers. A combustion system with such a structure would be substantially more reliable and less expensive than a conventional boiler/combustion system.
But in such a case, the temperature in the combustion chamber would become unacceptably high, such that the materials out of which the combustion chamber is constructed would be unable to withstand such temperatures. Moreover, the flue gases produced would have such a high temperature that they would not be able to be used directly to provide heat to the heat exchangers of a power system, especially if these heat exchangers are used to superheat vapor.
Separate combustion chambers, without internal waterwall cooling, have been used for the combustion of low quality fuels, particularly those with high water contents, such as biomass. However, even in these cases, the temperature of the flue gas produced is too high to be directly used in the heat exchangers of a power system.
Usually, in such cases, the hot flue gas is used to heat an intermediate heat carrying fluid, which in its turn is then used to provide heat to the heat exchangers of the power system. However such an arrangement results in the addition of substantial complications to the entire system.
Thus, there is a need in the art for a combustion apparatus that can utilize heat recovery steam generator (HRSG) or a heat recovery vapor generator (HRVG), which are relatively simple heat exchangers, in combustion systems allowing substantially less expensive and complex systems to be designed and constructed.